It had been 30 years since Thomas Jefferson likened the explosive issue of slavery expansion to “a fire bell in the night’” destined one day to sound “a death knell for the Union,” but the fulfillment of this grim prophecy seemed very much at hand in 1850.

Northern abolitionists had seemed to find their voice in the 1830s, and although it remained a minority voice, it grew loud and disruptive enough to spark genuine alarm in the slave states while making further political concessions to slavery increasingly problematic in the free states. Stung by abolitionist condemnations, enough northern Democrats abandoned their southern colleagues in 1847 to allow House passage of the Wilmot Proviso, which sought to keep slavery out of any territory acquired in the Mexican War. The slave-state/free-state balance in the Senate doomed the measure at that point, but population trends were changing. The slave states accounted for only 39% of the seats in the lower chamber when the admission of California as a free state, on Sept. 9, 1850, tipped the scales in the Senate as well.

With southern threats of disunion mounting, the measures comprising the “Compromise of 1850” undertook to mollify slaveholders in several ways, but none had more precisely the opposite effect in the free states than the incendiary Fugitive Slave Act, approved by Congress precisely 165 years ago, on Sept. 18, 1850.

Surely one of the most critically misguided pieces of legislation in U.S. history, it proved strikingly cruel, even for a pro-slavery measure. It forced the federal government to grossly overstep its bounds in defense of slavery at a time when anti-slavery sentiment was clearly on the rise, sparking outrage and defiance in the North, and, in turn, further deepening southerners’ suspicions that their rights could no longer be protected within the Union.

On its face, the new law simply set out to enforce the U.S. Constitution, specifically Article IV, Section 2, Clause 3, which declared that slaves did not become free simply by escaping to a free state and thus stipulated their return to their lawful masters. Yet, moving well beyond earlier efforts doing little more than lip service to this mandate, framers of the 1850 measure stripped northern courts of their authority over cases in which slaveholders sought return of their reputed runaways, vesting it solely in the hands of federal commissioners, blatantly encouraged to find in favor of the slaveholder by a compensation rate of $10 for each black person remanded South (compared to only $5 when the claim was disallowed). Precluding testimony by the alleged fugitives themselves, the act also compelled otherwise disinterested private citizens, upon threat of fine or imprisonment, to assist in their capture and return of the suspects.

With annual documented cases of runaways amounting to 1,000 or fewer out of a total slave population that stood at 3.2 million in 1850, some thought the slaveholders who demanded a stronger, more enforceable fugitive-slave law were either paranoid or simply overreacting, but there were some more tangible concerns in play. Escapes had mounted as the increasingly dynamic abolitionist contingent urged slaves to take flight, and, beyond that, the most compelling and credible indictments of slavery came not from northern whites acting on principle, but from runaways like Frederick Douglass and Henry Highland Garnet, who had experienced its cruelties themselves.

It was no mere coincidence that both Douglass and Garnet had made their escapes from Maryland. Historians Joseph R. Hummel and Barry R. Weingast have shown that the odds of permanent escape were so much greater in the border slave states that Delaware, Maryland and Missouri accounted for less than 6% of the total slave population of the slave states in 1850 but 36% of the runaways. Because the worst flight risks were also the most expensive to replace, able-bodied male slaves became a notably less attractive investment in the border states. As dramatically higher prices in the Deep South enticed more and more slaveholders in states like Delaware and Maryland to sell off their human property, there was little reason to expect their future representatives in Washington to maintain their attachments to the institution.

Yet if the Fugitive Slave Act of 1850 was expected to bolster slavery in any concrete fashion, there is little evidence it actually did. In fact, though the Fugitive Slave Act itself marked a low point in American legislative history, its very egregiousness ultimately helped to bring down the barbaric institution it was crafted to defend.

Runaway totals fell by scarcely 200 over the decade that followed, and the 330 persons returned to slavery barely matched the number of escapes from the Border States alone in 1860. These numbers seem doubly anemic when discounted against the massive backlash against the act in the northern free states, where it clearly accelerated rather than impeded the abolitionist movement, helping, among other things, to set a most receptive stage for Harriet Beecher Stowe’s Uncle Tom’s Cabin and to bolster support for the Underground Railroad. Even northern whites who had previously been little disposed to have African Americans in their midst now demanded stronger state protections for personal liberty. They even rose up in direct physical defiance in Boston, Oberlin and elsewhere, resisting federal intruders empowered to override the local justice system and abrogate their civil rights. The oft-invoked “slave power conspiracy,” now seemed intent on imposing its sinister expansionist will not just on remote territorial outposts, but on their very own, ostensibly “free,” communities, under the aegis and muscle of their own government. As what historian Eric Foner called “the most powerful exercise of federal authority within the United States” prior to the Civil War, the Fugitive Slave Law not only exacerbated the very fears and concerns on both sides that had thrown the Union into crisis in 1850 but, as would soon be evident, it further undermined the political fortunes of those who demanded it in this first place.

Like their counterparts a century later who were slow to realize the explosive potential of a steadily rising outcry for racial justice, the southern Democrats of 1850 had sorely underestimated a very real threat—not only to their interests, but ultimately to the Union itself. As the Fugitive Slave Act made clear, the shift in substantive northern priorities and the concomitant rise of a new public morality would be incompatible with any further extension of the physical and political reach of human bondage.

Historians explain how the past informs the present

James C. Cobb is Spalding Distinguished Professor of History at the University of Georgia and a former president of the Southern Historical Association.